It ought to be pretty obvious that banning drinking in a place is completely different from banning drunken louts from a place.
If you ban drinking in a place, it prevents people who aren’t louts but fancy a beer from having one, while doing absolutely nothing to prevent louts who are drunk from causing a nuisance (even if the drinking legislation were actually enforced against groups of rowdy chavs, which it won’t be).
If you actually want to stop drunken loutery, then you need to ensure that drunken louts are arrested, under the existing laws that provide a perfectly good arsenal of charges and punishments against rowdies, harrassers, disorderly conductors and affrayists. You don’t impose a new measure to punish the law-abiding.
Hence, the only two reasons to support Mr Johnson’s impending ban on drinking on the Tube are:
1) a belief that alcohol is inherently wrong and its consumption should be impeded wherever possible; or
Neither of these are attractive traits, so it’s worrying that the plan is seen as a vote-winner…
Side note: the ban appears to advertised as “making everyone’s journey more pleasant”. Since it will very clearly make journeys less pleasant for those who enjoy drinking while on a journey, this is clearly false advertising, and I’d urge everyone who sees such a poster to report it to the ASA.